On-frequency repeaters are known in the art, for amplifying an input signal without otherwise altering its frequency spectrum. In some cases, an on-frequency repeater may also employ various types of active circuitry in order to enhance the signal-to-noise (S/N) ratio, in addition to simply increasing the power level. A typical application of on-frequency repeaters is for improving wireless services within defined regions of a wireless network, where signal levels would otherwise be too low for satisfactory quality of service. For example, within a building, or a built-up urban area, signal attenuation, shadowing by buildings and/or hills, noise generated by various radio frequency sources, and multi-path effects can seriously degrade the quality of desired RF signals. In some cases, a wireless network provider may install a repeater in order to improve service in a region lying at an edge of the coverage area serviced by a base station, thereby effectively extending the reach of the base-station,
On-frequency repeaters are characterized by the fact that on input signal is amplified and retransmitted by the repeater at the same carrier frequency. For the purposes of the present invention, the term “on-frequency repeater” shall be understood to refer to any amplifier system that has this characteristic, irrespective of whether the system is used as part of an wireless communications network, or in any other context. The external input signal received by the repeater (e.g. from a base station or a subscriber's wireless communications device—WCD) can be represented by:Se=A(t)·Cos(ωt+m(t))  (1)Where A(t) is the amplitude information of the external input signal, ω is the carrier frequency and m(t) is the phase information of the carrier signal. In this case, the corresponding output signal radiated by the repeater can be represented by:So=G·A(t)·Cos(ω(t−δ)+m(t−δ))  (2)Where G is the repeater gain and δ is the time delay through the repeater at the carrier frequency ω.
It will be seen that the output signal (So) radiated by the repeater is a replica of the input signal received by the repeater, that has been amplified and subject to a time delay δ due to electrical delays within the repeater. Part of this delay is inherent to the amplification process, but is primarily caused by band-pass filters used in the repeater to prevent the unwanted amplification of signals outside the frequency band of interest. Generally this delay is inversely proportional to the bandwidth of the filters. The repeater gain (G) provides the increase in signal level that makes the repeater useful.
As will be appreciated, successful operation of the repeater requires that it provide sufficient system gain G to compensate for propagation losses. On the other hand, if the gain (in either the uplink or downlink paths) is too high, the repeater will radiate unnecessarily high signal power to mobile stations within its coverage area and/or the base station. In an environment in which there is more than one repeater in use, radiation of excessive signal power in the downlink path can cause interference (in the form of multiple overlapping coverage areas) with other subscribers. The same holds true for a single repeater radiating excessive power in the downlink path causing interference to other subscribers outside the intended coverage area of the repeater. Similarly, radiation of excessive signal power to the base-station may cause interference with other base-stations and/or other users of the wireless communications network.
Automatic Gain Controllers (AGCs) capable of controlling signal gain are known in the art. Typically, AGCs are implemented as analog RF or IF circuits, in which a variable gain amplifier (VGA) is used to amplify the analog RF signal. The VGA is normally controlled by a control signal derived from a measured signal parameter. In open-loop systems, the measured signal power is normally the output signal power, that is, the power level of the RF signal being supplied to an antenna for transmission. Thus, for example, in the downlink path (from the base station to the mobile station within the local coverage area) the power level of the downlink signal supplied to the coverage area antenna is monitored by a feed-back path and used to generate the VGA control signal. This arrangement enables the VGA to provide a variable gain, so that the effective radiated power (ERP) of the downlink RF signal is approximately constant, in spite of variations in the received power of the downlink RF signal from the base station. A directly analogous operation is also performed in the uplink path, so that the effective radiated power (ERP) of uplink RF signals transmitted to the base station will be approximately constant, even with wide variations in the received power of the downlink RF signal from the base station.
A limitation of this arrangement is that the signal power level measured at the repeater output represents the total power within the signal path, rather than the power levels of desired traffic signals within it. In particular, the signal path will normally have a bandwidth of 25 MHz, or more, and contain multiple channels. In many wireless communications systems, such as Time Division Multiple Access (TDMA), Advanced Mobile Phone Service (AMPS) and the 15-95 CDMA (Code Division Multiple Access) system, the signal power within the control channel(s) will remain approximately constant, but the power level in each of the data channels will fluctuate widely in accordance with variations in the signal traffic. However, the AGC cannot discriminate between a power level increase due to increased traffic in a data channel, and power level increases due to any other cause (such as an increase in the received signal power). In all cases, the AGC will respond to increased output power by reducing gain. Where the measured power level increase is due to increased data channel traffic, however, this produces the undesired result that the AGC has responded to the increased data traffic by attenuating the signal power.
Applicant's co-pending U.S. patent application Ser. No. 10/359,096 filed Feb. 6, 2003 provides an Intelligent Gain Control method and system which operates by identifying and isolating a desired narrow band channel within a broadband signal path. The gain of the broadband signal path is then controlled to maintain the ERP of the isolated channel substantially constant. Thus broadband gain control is implemented based on narrow band power levels of desired channels within the broadband signal path. This avoids the limitation of prior art AGC systems, in which path gain is controlled based on the total power level (of all of the traffic) within the signal path.
A limitation of this approach is that the IGC can become unreliable if signal traffic within the isolated narrow band channel is discontinuous. In the system of U.S. patent application Ser. No. 10/359,096, this problem is addressed by hunting for and isolating a control channel within the signal path as the desired channel for controlling gain of the signal path. Use of a control channel for gain control improves reliability because such channels almost always carry a valid signal, even when little or no subscriber data traffic is being conveyed through the network. For many common wireless communications systems, such as TDMA and AMPS, such control channels can be readily isolated. However, for some communications systems, such as mixed format (e.g. Motorola's proprietary integrated Digital Enhanced Network-iDEN) communications systems, the control channel signaling may be discontinuous. In other systems, such as the Global System for Mobile Communications (GSM) the control channel signaling may not be readily distinguishable from other signal traffic or indeed from background noise.
Accordingly, a method and system capable of reliably recognizing desired narrow band signals within a broadband signal path remains highly desirable.